The world reliance on petroleum and natural gas has reached an era where the supply and demand have become critical. These circumstances make the need for innovative energy and environmental technologies essential to mediate climate change, reduce greenhouse gas emissions, reduce air and water pollution, promote economic development, expand energy supply options, increase energy security, decrease U.S. dependence on imported oil, and strengthen rural economies. It is now essential that energy conversion systems and processes be introduced and commercialized that can employ alternative sources of energy in an environmentally benign manner at economic costs, and can transform abundant carbonaceous material resources into clean, affordable, and domestically-produced renewable fuels and high-value products.
New technology is needed in order to exploit alternative sources of energy and feedstock for sustainable economic development in an energy efficient manner while maintaining a clean and unpolluted environment. The needed technologies must be sufficiently flexible, thermally efficient, energy integrated, environmentally clean and cost effective to enable the use of abundant carbonaceous materials for the production of clean and cost effective energy. Further, decreasing world reserves and diminishing availability of crude oil have created considerable incentive for the development and use of alternative fuels. In recent years, the ever increasing value of fossil hydrocarbon liquids and gases has directed research, development, deployment, and commercialization to the possibilities of employing carbonaceous materials for fuel purposes. In particular, attention has been focused on thermochemical conversion of carbonaceous materials.
Reaction vessels containing a fluidized bed of bed material are well suited to effectuate thermochemical processes to convert carbonaceous materials into product gases. A fluidized bed is formed when a quantity of a particulate bed material is placed under appropriate conditions in a reactor vessel to cause a bed material to behave as a fluid. This is usually achieved by the introduction of pressurized steam, carbon dioxide, oxygen-containing gas, and/or any other gases, or vapors, to flow through the particulate bed material. This results in the bed material then having many properties and characteristics of normal fluids.
Converting a carbonaceous material, such as municipal solid waste (MSW), into a product gas by the use of a fluidized bed reactor poses an exceptionally difficult challenge. This is inherently due to the inert contaminants that are present within the MSW. MSW, commonly known as trash or garbage in the United States is a waste type comprised of everyday items that are discarded by the public. Inert contaminants cannot be converted into product gas, however other portions of a MSW carbonaceous material can be converted into product gas. Instead, the MSW inert contaminants build-up and accumulate within the quantity of bed material contained within the reactor thus inhibiting and undermining the ability of the reactor to effectuate appropriate fluidization of bed material for any thermochemical process to take place at all.
In applying the classification of gas/solid systems according to Geldart (D. Geldart, Powder Techn. 7, 285-293, 1973), if a fluidized bed contains mostly easily fluidized Geldart Group B bed material, fluidization will diminish if Geldart Group D solids (inert contaminants) accumulate within the fluidized bed. Geldart Group D solids may be the inert feedstock contaminants that are introduced with the MSW. Or the Geldart Group D solids may be generated through agglomeration of Geldart Group A or Geldart Group B solids. Nonetheless, a fluidized bed of a mean bed particle characteristic of Geldart Group B solids may become defluidized by buildup or accumulation of comparatively larger, coarser and/or heavier Geldart Group D solids that are introduced to the fluidized bed from an external source, such as with MSW. Defluidization may also be caused by predictable agglomeration or growth of one or more types of Geldart solids groups fusing or binding or growing together to form larger Geldart solids groups.
Defluidization may be caused by unpredictable and unavoidable buildup of larger Geldart particles, in comparison to the mean bed particle characteristic, introduced to the fluidized bed. The accumulation of Geldart Group D solids in a fluidized bed having a mean bed particle characteristic of Geldart Group B solids often results in defluidized or stagnant zones in the fluidized bed and in turn demanding an increase in fluidization velocity to maintain fluidization quality.
Often times when a carbonaceous material feedstock possessing silicon, potassium, chloride, sodium and/or alkali earth metals within the ash, the softening or melting temperatures of these compounds may be less than the operating temperature of the thermochemical reaction environment. And as a result, the growth and accumulation of agglomerates within the fluidized bed transitions from proper fluidization to possible economically detrimental defluidization leading to unscheduled process termination and shut down.
Various different methods of agglomeration have been described in scientific literature. Specifically, Pietsch, W. Size Enlargement by Agglomeration (New York: John Wiley & Sons, 1991) puts forth various different binding methods of agglomeration. Perhaps the most significant types of binding mechanisms relevant to fluidized bed agglomeration in applications for generation of product gas from carbonaceous materials possessing elevated silicon, potassium, chloride, sodium and/or alkali earth metals within the ash are solid bridges such as mineral bridges, sinter bridges, chemical reaction, partial melting, hardening binders, crystallization or deposition of suspended colloidal particles. Further, agglomeration may be compounded by the presence of any of the aforesaid binding mechanisms together with interlocking of two or more fluidized bed particulates together thus eventually increasing the mean particle size of the bed leading to defluidization.
Removal of accumulation of agglomerates, or removal of accumulation of larger size Geldart type solids, in comparison to the fluidized bed mean bed Geldart particle group, and introduced to the fluidized bed from an external source, in many applications is impossible to do in-situ. In many instances, buildup of larger Geldart solid classifications within a fluidized reaction environment of lesser sized Geldart solids, say accumulation of Geldart D solids in a fluidized bed environment of Geldart type B solids, requires process interruption and periodic termination of operation for cleaning.
Fluidized beds typically usually have a mean bed particle characteristic of Geldart Group B solids, generally with no overlap of Geldart Group A or Geldart Group D solids. It is therefore desirable to be able to remove Geldart Group D solids which may accumulate within the fluidized bed of Geldart Group B solids to maintain continuous operation of the fluidized bed. Further, some fluidized bed systems have a mean bed particle characteristic of Geldart Group A solids, generally with no overlap of Geldart Group B or Geldart Group D solids. It is also therefore desirable to be able to remove any Geldart Group B or Geldart Group D solids which may accumulate within the fluidized bed of mostly Geldart Group A solids to maintain continuous operation of the fluidized bed. Therefore, a need exists for a new fluidized bed process that is better suited to operate on a continuous and uninterrupted basis by accommodating size and density classification of smaller type Geldart solids for recycle back to the fluidized bed while removing solids of comparatively larger Geldart type from the system.